1. Field
This disclosure relates to a battery system installed in a vehicle that is capable of traveling using power from at least one of an engine and a motor-generator.
2. Description of Related Art
Japanese Patent No. 5149989 discloses a hybrid vehicle that is capable of switching between electric travel, in which an engine is stopped so that the vehicle travels using power from a motor-generator, and hybrid travel, in which the vehicle travels using power from both the engine and the motor-generator. In this hybrid vehicle, either a charge depleting (CD) mode or a charge sustaining (CS) mode is selected as a control mode in accordance with a state of charge (SOC) of an electricity storage device that exchanges power with the motor-generator. The CD mode is a control mode in which driving of the engine (hybrid travel) is prohibited in order to maintain the SOC of the electricity storage device within a predetermined control range. The CS mode is a control mode in which driving of the engine (hybrid travel) is permitted in order to maintain the vehicle SOC within the predetermined control range. As a result, greater priority is given to electric travel over hybrid travel in the CD mode than in the CS mode.
In the hybrid vehicle disclosed in Japanese Patent No. 5149989, the CD mode is selected until the SOC of the electricity storage device falls to a predetermined value, and when the SOC of the battery falls to the predetermined value, the CD mode is switched to the CS mode.
When the electricity storage device is a battery pack formed by connecting a plurality of battery units in series, differences may occur among the SOCs of the respective battery units due to various factors such as individual differences among the respective battery units and variation in a degree of deterioration therein. Therefore, to prevent over-discharge of the respective battery units, an overall SOC (also referred to hereinafter as a “vehicle SOC”) of the electricity storage device may be calculated in consideration of the SOC (also referred to hereafter as a “minimum unit SOC”) of the battery unit having the lowest SOC. For example, the vehicle SOC may be calculated such that when the minimum unit SOC is at a lower limit value, the vehicle SOC also takes a lower limit value.
In the hybrid vehicle disclosed in Japanese Patent No. 5149989, however, when the effect of the minimum unit SOC on the vehicle SOC is set to be excessively large in order to prevent over-discharge, the vehicle SOC is calculated at a lower value than necessary relative to an actual overall amount of power stored in the electricity storage device at a point where the vehicle SOC falls to a predetermined value. As a result, the CD mode may be switched to the CS mode earlier than necessary, leading to a reduction in an electric travel distance.